High purity aluminum oxide or alumina powder can be used to make; translucent tubes for high-pressure sodium lamps, sapphires for watch covers, high-strength ceramic tools, abrasives for magnetic tape, manufacturing light emitting diodes as a substrate for GaN, silicon microchip wafers for optic-electronics, windows and cowls for aircrafts, protective windows for car headlamps, cell phones and other electronic devices, stop signals, surgery scalpels, micro-optical elements of medical fiber-optic probes, optical scanners for bar codes, ultraviolet CD and DVD optical systems, prisms, lenses, optical plates, optical systems of visual and IR diapasons, cell phone, mobile devices and fiber-optic system display windows, equipment for chemical manufacturing in aggressive and high-temperature environments: tubes, crucibles, funnels, chemical glassware, abrasives, battery components, bearings and jewelry stones.
Currently the most common methods of making high purity alumina for manufacturing Sapphire for LED substrates are aluminum-ammonium-sulfate thermal decomposition, aluminum-ammonium-carbonate thermal decomposition and aluminum-isopropoxide hydrolyzation. The high purity alumina is then used in the Verneuil process to make crackle or compressed into densified pucks, granules or beads for melting in a sapphire ingot furnace.
These processes are well known. A low cost process, which uses less energy, is needed to make high purity alumina for manufacturing low-cost sapphire, such as can be used for sapphire substrates for LEDs.
Most past work in the field of alumina purification used aluminum trihydrate, Bauxite, gibbsite, aluminum oxides or ores containing aluminum oxide as the starting raw material for the process. Using aluminum as the starting raw material for manufacturing high purity aluminum oxide is very difficult due to the fact that it is difficult to control the reaction rate of the acid with the aluminum. High purity aluminum reacts very slowly with acid and then can very quickly accelerate into a very quick exothermic reaction. At each step of the process the feedstock can be contaminated by the reaction vessel, furnace or holding container. It is very important to use the correct materials and to control the reaction and temperature at each step to prevent contamination in the process in order to reach a high purity with a low cost. In the past it has been difficult to dissolve high purity aluminum economically in acid due to the fact that the higher the purity of the aluminum the slower the reaction with the acid. Use of aluminum with very high surface area will increase costs and potentially cause a runaway reaction due to the exothermic reaction.
Most of the past research done on using acids to process ores high in aluminum content into aluminum oxide was done to make a feedstock for producing primary aluminum. These processes are concerned with reaching the purity limits for the Hall-Heroult process but are not focused on reaching 4-6N purity requirements require to make a sapphire grade LED substrates or alumina for other high purity applications. Conventional high purity alumina typically has the following impurities: Na<10 ppmw, Fe<5 ppmw, Si<10, Ti<3, Mg<2, Ca<2, with 99.99% or 4N aluminum oxide purity. 5N purity alumina as feedstock for sapphire ingots can increase the yield and through put for the sapphire ingot making process and the LED manufacturing process.